What kind of calls? VoIP calls can be encrypted relatively easily, there are loads of ways to do it.

Landline phones? I'm not sure, but I believe there are devices out there to do it. Or you could roll your own with a pair of old modems.

Cell phone? Short answer: No. Long answer: In the U.S. at least, the carriers can't encrypt the calls because CALEA requires them to accommodate law enforcement wiretaps. Plus, encryption would add overhead to their precious bandwidth. Plus, the lossy compression performed on a digital cell phone voice stream mangles normal audio so as to be just a hair above completely unintelligible thus encrypted audio certainly wouldn't make it through.

But as a first concept, if delay is not really a problem: reversed speech is not clearly understandable and might make it through the compression. Though it's not as good as real encryption, it's something that might work (I haven't tested it). And perhaps even a bit better delay-wise: splitting the audio-stream in chunks (of a few hundred ms or even a few seconds), can lower the delay but I guess the shorter each chunk of reverse audio, the better it could become to understand everything.

For shorted messages, it might work to just send audio files through MMS; and those audio files can of course be altered in any way you want, I guess.

So the most secure option would be to use VoIP, which can be encrypted without problem.

I think there are civilian variants. You can't buy the Type I certified units unless you're a government contractor or the government themselves. I have no idea whether or not the civilian variants have any sort of backdooring.

There is no encryption technology available to the general population that will prevent government / law enforcement from being able to read the data stream. Depending on who wants to read it, it's trivial.

Before everyone gets too excited about encryption, I'd suggest you look up civilian cryptography guru Bruce Schneier and read what he has to say on the subject.

Short version: there was a time in the early days of civilian cryptography when people believed encryption was a 'magic security dust' (his term) that people could sprinkle on problems and make the bad stuff go away. As civilian experts spent more time exploring detailed real-world problems, they found that security protocols are even more important. It doesn't matter how good your lock is if your bank vault has a big plate glass window.

Real security is a hard problem, and many years of expert research have shown there's no product you can buy that will make it stop being a hard problem.

WRT cell phone encryption, the first and most obvious problem is, "how do you plan to transfer the keys that allow the people at the other end to decrypt your call?" An unencrypted phone call is probably not the best option. On another track, if the keys live in hardware, anyone who gets physical access to any phone you can call has just broken your security.

WRT the recent disclosures about PRISM, encrypting the conversation is irrelevant. The NSA has been collecting metadata about the calls.. which numbers you called, how long each call lasted, when you made the calls, where you were when you made each call, etc. Your actual conversation could be in a pdigin of Navajo, Teochow, and Basque for all it matters. The stuff the NSA has collected is inherent in making the phone system operate.

When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.